The present invention relates to a color image electrophotographic apparatus having a photosensitive drum and an intermediate transfer body and more particularly to a color image electrophotographic apparatus such as a color laser beam printer having a photosensitive drum and an intermediate transfer body in which the color image can be obtained in accordance with the form by overlapping the different color images on the intermediate transfer body.
The present invention relates to a color laser beam printer having a photosensitive body belt and an intermediate transfer body in which the photosensitive body belt has at least one joint portion.
In the color laser beam printer using an electronic-photography technique, the color image is formed by overlapping the different color images. In this color laser beam printer, it causes an important problem about the color offset between one color and another color.
In the color laser beam printer including a photosensitive body and an intermediate transfer body, first of all the laser beam modulates with an image data of a first color and exposes against the photosensitive body and a latent image is formed on the photosensitive body.
In the present invention, the first place color indicates a color which is requested firstly for forming the color image from a host (an image data generating means) and the second place color indicates a color which is requested at second for forming the color image from the host and so on. In other words, the first color indicates the first place color data which is sent firstly from the host and the second place color indicates a second color data which is sent secondary from the host.
The latent image is developed by a development means of the first color and the toner image is obtained. Such an obtained toner image is transferred or shifted on the intermediate transfer body. In secession, the latent image of the second color is formed on the surface of the photosensitive body using the similar method stated above for forming the first place color and the latent image of the second place color is developed means of the second color and the toner image is obtained.
Such an obtained toner image of the second color is overlapped or transferred on the toner image of the first place color of the intermediate transfer body. Thereby, the color image can be formed on the intermediate transfer body. As occasion demands, the toner image of a third place color and the toner image of a fourth place color are overlapped or transferred on the intermediate transfer body and the respective color image is formed on the intermediate transfer body.
After that the color image is overlapped or transferred collectively on a printing paper which is sent by a printing paper transportation means and the color image can be obtained. Accordingly, it becomes an important problem about a color match on the intermediate transfer body.
Taking measurement to meet the above stated problem, the following countermeasures have been proposed.
For the improvement of the color match, in a conventional color laser beam printer, for example disclosed in Japanese Patent Laid-Open No. 195687/1987, a photosensitive body and a transfer body are driven at a constant speed by synchronizing a basic clock signal and an exposure timing and a transfer timing of each color are carried out using the above stated basic clock signal.
Further, for the improvement of the color match, in another color laser beam printer, for example disclosed in Japanese Utility Model Laid-Open 155147/1988, a control means comprises a pulse from an encoder installed on a drive motor as the printing paper transportation means and a pulse from a sensor installed on a midway of a printing paper transportation passage. In this prior art, an exposure timing and a transfer timing of each color are carried out using the above stated pulse signal.
However, with the above stated control means provided on the conventional color laser beam printer, in a case of the color laser beam printer disclosed Japanese Patent Laid-Open No. 195687/1987 (the former prior art), there is not considered fully about the exposure start timing control for each color of the image data. Accordingly, it is difficult to form the toner image for each color at an accurate position, as a result it causes the color offset.
Besides, in the color laser beam printer disclosed in Japanese Utility Model Laid-Open 155147/1988 (the latter prior art), since the transportation position of the printing paper is detected indirectly, the change in the mechanical relationship between the encoder and the printing paper transportation means, especially the peripheral length of the belt type transportation means changes according to the environment change in the temperature and the humidity, the aged deterioration and the exchange by the maintenance etc., accordingly as a result there causes a problem about the color offset.
Further, in a case that the belt member is employed as the photosensitive body, as the belt member to be used it is desirable to employ the endless type belt. However, from the aspect of the cost in manufacturing, it is used frequently the belt member for the photosensitive body having the joint portion in comparison with the endless type belt.
In the case of the belt member for the photosensitive body having the joint portion, since at the joint portion the electrostatic latent image is not formed thereon, it is necessary to manage the joint portion of the belt member for the photosensitive body.
FIG. 8 is a control timing chart showing the conventional color laser beam printer structure. The control timings are an exposure timing and a transfer timing of each color. In this conventional color laser beam printer, the printing processing is carried out according to a certain fixed timing.
FIG. 8 shows the relationships between the joint portion position of the photosensitive body, an intermediate transfer drum (an intermediate transfer body) sensor signal and a vertical synchronizing signal (VSYNC-N 30).
For example, after the fixed time tm as the standard of the signal of one printing paper sensor, when the printing is controlled so as to send the vertical synchronizing signal (VSYNC-N 30) as an image data requirement signal to the image data generating means as the host, the joint portion position of the photosensitive body belt is slipped every time of the rotation.
As shown a point (p) in FIG. 8, some time the joint portion of the photosensitive body drum may enter into the image area of the intermediate transfer body.